Secondary battery

Abstract

A secondary battery according to the present invention comprises a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the positive electrode and the negative electrode are wound with the separator interposed therebetween, the positive electrode is provided with a strip-shaped positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector, the positive electrode has a first region including one end in a short side direction of the positive electrode and a second region other than the first region, the first region has an exposed portion, of the positive electrode current collector, partially provided at one or more locations along the longitudinal direction of the positive electrode current collector, the exposed portion does not have a positive electrode mixture layer from the one end in the short side direction to the second region, the positive electrode mixture layer includes a positive electrode active material and a binder, and a loop stiffness load value L1 of the first region is smaller than a loop stiffness load value L2 of the second region.

Description

secondary battery Cross-reference of related applications

[0001] This disclosure claims priority with respect to Japanese Patent Application No. 2024-169317, filed with the Japan Patent Office on 27 September 2024, and the entirety of the said patent application is incorporated herein by reference.

[0002] This disclosure relates to secondary batteries.

[0003] Patent Document 1 proposes a method for manufacturing an electrode plate for an energy storage device, in which a mixture layer is formed on a current collector by discharging a mixture slurry from a discharge nozzle corresponding to each of the discharge regions into a plurality of discharge regions extending along the length of the current collector, wherein the positions of the discharge regions are set such that a part of each of the plurality of discharge regions overlaps with a part of an adjacent discharge region when viewed from the length of the current collector, the overlapping portion has a length in the width direction of the current collector of 8 mm or less, and an uncoated portion is provided in at least one of the discharge regions by intermittently discharging the mixture slurry.

[0004] Patent Document 2 proposes "an electrode plate for an energy storage device, comprising a substantially rectangular current collector and an active material layer provided on at least one surface of the current collector, wherein the current collector has a blank portion to which electrode leads are connected at one end in the width direction in a part of the longitudinal region, and in the region where the active material layer is provided, the elastic modulus of a first region adjacent in the width direction to the blank portion is greater than the elastic modulus of a second region adjacent in the longitudinal direction to the region occupied by the blank portion and the first region."

[0005] Patent No. 6965162, Patent No. 6821595

[0006] In Patent Documents 1 and 2, electrode leads are connected to the uncoated (or blank) portion of the composite layer. When the electrode is wound, the curvature of the uncoated portion to which the electrode leads are connected decreases, while the curvature near the uncoated portion of the composite layer increases. Therefore, the difference in curvature is greater in the region along the longitudinal direction of the electrode that includes the uncoated portion of the composite layer than in the region along the longitudinal direction of the electrode that does not include the uncoated portion, and differences in surface pressure applied to the composite layer are likely to occur. As a result, when the charge-discharge cycle is repeated, the electrochemical reaction inside the battery becomes non-uniform, and the decrease in discharge capacity becomes greater.

[0007] One aspect of the present disclosure relates to a secondary battery comprising a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the positive electrode and the negative electrode are wound around the separator, the positive electrode comprises a strip-shaped positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector, the positive electrode has a first region including one end in the short direction of the positive electrode and a second region other than the first region, the first region has one or more exposed portions of the positive electrode current collector provided partially along the longitudinal direction of the positive electrode current collector, the exposed portions do not have the positive electrode mixture layer from the one end in the short direction to the second region, the positive electrode mixture layer comprises a positive electrode active material and a binder, and the value of the loop stiffness load L1 of the first region is smaller than the value of the loop stiffness load L2 of the second region.

[0008] According to this disclosure, the difference in surface pressure applied to the positive electrode mixture layer in a secondary battery can be reduced, thereby improving the cycle characteristics. Novel features of the present invention are described in the appended claims, but the present invention, both in terms of structure and content, and in conjunction with other objects and features of the present invention, will be better understood by the following detailed description in conjunction with the drawings.

[0009] This is a schematic cross-sectional view of a secondary battery according to one embodiment. This is a schematic plan view of the positive electrode according to one embodiment. This is a schematic plan view of the negative electrode according to one embodiment.

[0010] The embodiments of this disclosure will be described below with examples, but this disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be given as examples, but other numerical values ​​and materials may be applied as long as the effects of this disclosure are obtained. In this specification, the description "numerical value A to numerical value B" includes numerical value A and numerical value B, and can be read as "greater than or equal to numerical value A and less than or equal to numerical value B". In the following description, when lower and upper limits of numerical values ​​relating to specific physical properties or conditions are given as examples, either of the given lower limits and either of the given upper limits can be arbitrarily combined, as long as the lower limit is not greater than or equal to the upper limit.

[0011] Furthermore, this disclosure encompasses any combination of matters described in two or more claims, which may be arbitrarily selected from the multiple claims set forth in the attached claims. In other words, any combination of matters described in two or more claims, which may be arbitrarily selected from the multiple claims set forth in the attached claims, is possible, provided that no technical inconsistency arises.

[0012] Secondary batteries include lithium-ion secondary batteries, lithium metal secondary batteries, and non-aqueous electrolyte secondary batteries such as solid batteries containing gel electrolytes or solid electrolytes. In other words, secondary batteries may be liquid-type secondary batteries containing an electrolyte solution, or all-solid-state secondary batteries containing a solid electrolyte.

[0013] The secondary battery according to this disclosure comprises a strip-shaped positive electrode, a strip-shaped negative electrode, an electrolyte, and a separator disposed between the positive and negative electrodes. The positive and negative electrodes are wound around each other via the separator. In other words, the secondary battery comprises a wound-type electrode group. The cross-sectional shape of the wound-type electrode group perpendicular to the winding axis may be, for example, circular or elliptical, and the outer shape may be, for example, cylindrical, but is not limited to these.

[0014] [Positive Electrode] The positive electrode comprises a strip-shaped positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector. The positive electrode mixture layer may be in the form of a film. The positive electrode has a positive electrode current collector and a positive electrode mixture layer formed (or supported) on a portion of the surface of the positive electrode current collector. Specifically, the positive electrode has a first region (positive electrode edge) including one end in the short direction of the positive electrode and a second region (positive electrode main part) other than the first region. The first region has exposed portions of the positive electrode current collector, one or more of which are partially provided along the longitudinal direction of the positive electrode current collector. The exposed portions of the positive electrode current collector do not have a positive electrode mixture layer from one end in the short direction to the second region. Multiple exposed portions of the positive electrode current collector may be provided intermittently along the longitudinal direction of the positive electrode current collector. The exposed portions of the positive electrode current collector may be partially exposed portions that are generally rectangular and have a predetermined width in the longitudinal direction of the positive electrode current collector.

[0015] The positive electrode mixture layer is composed of a positive electrode mixture. The positive electrode mixture contains a positive electrode active material and a binder as essential components, and may contain optional components such as a conductive additive and a thickener. Since the positive electrode mixture contains a positive electrode active material as an essential component, the positive electrode mixture layer may also be called the positive electrode active material layer. The positive electrode mixture layer is supported on one or both surfaces of the positive electrode current collector.

[0016] In this disclosure, the positive electrode satisfies the following condition A.

[0017] Condition A: The loop stiffness load value L1 in the first region is smaller than the loop stiffness load value L2 in the second region.

[0018] The loop stiffness satisfying L1 < L2 can also be said to mean that the first region is more flexible than the second region, or that the stiffness of the first region is less than the stiffness of the second region.

[0019] When the above condition (A) is met, the difference in surface pressure applied to the mixture layer is suppressed. This is thought to be because when the loop stiffness satisfies L1 < L2, the curvature difference of the first positive electrode mixture portion becomes close to the curvature difference of the second positive electrode mixture portion.

[0020] In a wound electrode group where the positive electrode lead is connected to the exposed portion of the positive electrode current collector, it is believed that the distribution of strain in the electrode group becomes more uniform by making the loop stiffness L1 of the first positive electrode mixture, which is prone to generating a relative difference in curvature, smaller than the loop stiffness L2 of the second positive electrode mixture. By making the distribution of strain in the electrode group more uniform, the winding ability of the electrodes is improved, and the electrochemical reaction within the electrode group becomes more uniform, allowing the capacity to be maintained at a high level during the charge-discharge cycle.

[0021] The ratio L1 / L2 between the value of the loop stiffness load in the first region and the value of the loop stiffness load in the second region is preferably, for example, 0.5 or more and less than 1.0, more preferably 0.6 or more and less than 1.0, and may also be 0.6 or more and 0.95 or less, or 0.6 or more and 0.9 or less, or 0.6 or more and 0.8 or less.

[0022] The L1 / L2 ratio can be controlled by, for example, the composition of the positive electrode mixture constituting the first and second positive electrode mixture portions, the physical properties of the positive electrode active material contained in the positive electrode mixture (constituent elements, hardness, particle fracture strength, etc.), the density of the first and second positive electrode mixture portions, and the mass of the positive electrode mixture layer per unit area of ​​the surface of the positive electrode current collector in the first and second regions.

[0023] For example, the L1 / L2 ratio may be controlled by the electrode density of the first and second regions. Specifically, the electrode density D1 of the first region may be made smaller than the electrode density D2 of the second region. Note that electrode density is synonymous with the density of the positive electrode mixture layer, where the electrode density D1 of the first region is the density of the first positive electrode mixture, and the electrode density D2 of the second region is the density of the second positive electrode mixture.

[0024] The ratio of the plate density D1 in the first region to the plate density D2 in the second region, D1 / D2, may be, for example, 0.7 or more and less than 1.0, 0.8 or more and less than 1.0, 0.8 or more and 0.99 or less, or 0.85 or more and 0.99 or less.

[0025] The electrode plate density D1 in the first region and the electrode plate density D2 in the second region are, for example, 3.3 g / cm³ each, independently. ３ The above is 3.4 g / cm³. ３The above is also acceptable, 3.5 g / cm³. ３ The above is also acceptable. The upper limits of the electrode densities D1 and D2 are not particularly limited, but for example, 3.8 g / cm³ ３ That's fine.

[0026] Furthermore, the L1 / L2 ratio may be controlled, for example, by the mass of the positive electrode mixture layer distributed per unit area of ​​the surface of the positive electrode current collector in the first and second regions. Specifically, the mass M1 of the positive electrode mixture layer distributed per unit area of ​​the surface of the positive electrode current collector in the first region may be smaller than the mass M2 of the positive electrode mixture layer distributed per unit area of ​​the surface of the positive electrode current collector in the second region.

[0027] The ratio M1 / M2 of the masses M1 and M2 of the positive electrode mixture layers arranged per unit area on the surface of the positive electrode current collector in the first and second regions is, for example, 0.7 or more and less than 1.0, 0.8 or more and less than 1.0, 0.8 or more and 0.99 or less, or 0.85 or more and 0.99 or less.

[0028] To increase the energy density of secondary batteries, the positive electrode mixture layer is increasingly being filled to be thicker and denser. As the positive electrode mixture layer becomes thicker and denser, more sophisticated measures are needed to counteract the difference in surface pressure applied to the positive electrode mixture layer. In other words, when the first region has one or more or intermittently multiple exposed portions of the positive electrode current collector along the longitudinal direction of the positive electrode current collector, it is important to suppress the difference in surface pressure applied to the positive electrode mixture layer. In such circumstances, forming a positive electrode mixture layer with the above configuration that satisfies the above conditions is an effective measure to suppress the difference in surface pressure of the positive electrode mixture layer.

[0029] In the first and second regions, the masses M1 and M2 of the positive electrode mixture layer disposed per unit area of ​​the surface of the positive electrode current collector are, independently, for example, 240 g / m². ２ The above is also acceptable, 260 g / m ２ The above is also acceptable, 280 g / m ２The above may also be applicable. The greater the mass of the positive electrode mixture layer disposed per unit area of the surface of the positive electrode current collector, the greater the effect obtained by satisfying the above conditions. That is, the effect of suppressing the curvature difference of the first positive electrode mixture portion becomes more prominent. In the first region and the second region, the masses M1 and M2 of the positive electrode mixture layer disposed per unit area of the surface of the positive electrode current collector are each independently, for example, 240 g / m ２ or more and 350 g / m ２ or less may also be within the following range.

[0030] Incidentally, the positive electrode current collector is in the form of a sheet and has one first surface and a second surface on the opposite side thereof. The "unit area of the surface of the positive electrode current collector" means the "unit area of the first surface of the positive electrode current collector" or the "unit area of the second surface of the positive electrode current collector". In other words, the mass of the positive electrode mixture layer disposed per unit area of the surface of the positive electrode current collector means the mass of the positive electrode mixture layer disposed per unit area of one surface of the positive electrode current collector.

[0031] The thicknesses of the positive electrode mixture layer (the thickness T1 of the first positive electrode mixture portion and the thickness T2 of the second positive electrode mixture portion) may each be, for example, 30 μm or more, 60 μm or more, or 90 μm or more. The greater the thickness of the positive electrode mixture layer, the greater the effect obtained by satisfying the above conditions. That is, the effect of suppressing the curvature difference of the first positive electrode mixture portion becomes more prominent. The thickness of the positive electrode mixture layer may be, for example, within the range of 50 μm or more and 80 μm or less.

[0032] <Method for Measuring Loop Stiffness> First, cut out five or more ribbon-shaped electrode plate samples of the same size from the first and second regions of the positive electrode. Preferably, the length of the sample should be selected from 90 to 180 mm and the width from 3 to 25.4 mm. Then, measure the load of each loop stiffness using a commercially available measuring device. Specifically, overlap the ribbon-shaped electrode plate samples with the same side faces facing each other to form a loop, and secure the overlapped ends with a chuck. Then, press a flat indenter against the tip of the loop (opposite side of the chuck) to measure the load. With an indentation speed of 2 mm / second, the load (rebound force) when the distance between the chuck and the indenter is 10 mm is taken as the load value of the loop stiffness. Obtain five or more L1 and L2 values ​​for each, and calculate the average value.

[0033] As a commercially available measuring device, for example, the "Loop Stiffness Tester (trademark registered) No. 581" manufactured by Toyo Seiki Seisakusho Co., Ltd. can be used. The measurement environment for loop stiffness is room temperature, but an ideal environment is, for example, 23°C and 50% relative humidity. The method for measuring loop stiffness is a well-known method adopted in many publications, such as Japanese Patent Publication No. 2024-107327 and Japanese Patent Publication No. 2024-024446.

[0034] Furthermore, the ratio T1 / T2 of the thickness T1 of the positive electrode mixture layer in the first region (hereinafter also referred to as the "first positive electrode mixture portion") to the thickness T2 of the positive electrode mixture layer in the second region (hereinafter also referred to as the "second positive electrode mixture portion") is preferably 0.97 or more and 1.03 or less.

[0035] The fact that T1 and T2 satisfy 0.97 ≤ T1 / T2 ≤ 1.03 means that, considering manufacturing variability, T1 = T2 (i.e., T1 / T2 = 1).

[0036] The positive electrode mixture layer can be formed, for example, by dispersing a positive electrode mixture containing particles of the essential component, the positive electrode active material, a binder, and optional components (such as conductive additives), in a dispersion medium, applying the slurry to the surface of the positive electrode current collector, and drying it. The dried coating may be rolled if necessary. The positive electrode mixture layer may be formed on one surface of the positive electrode current collector or on both surfaces. As the dispersion medium for the positive electrode slurry, N-methyl-2-pyrrolidone (NMP), cyclohexanone, alcohols, ethers, etc., can be used.

[0037] The binder may include, for example, a fluorinated polymer. Fluorinated polymers can exhibit high binding strength. A fluorinated polymer is a general term for polymers that have fluorine atoms (F) bonded to carbon atoms that constitute the main chain. Because fluorine atoms have a small atomic radius and polarizability, the carbon-fluorine bond is stable, and they have crystalline properties, which can also achieve excellent heat resistance, weather resistance, and chemical resistance.

[0038] The fluorine-based polymer may be, but is not limited to, polyvinylidene fluoride polymers or polytetrafluoroethylene polymers. In particular, the fluorine-based polymer preferably contains polyvinylidene fluoride polymers, and the polyvinylidene fluoride polymer may account for 50% or more by mass, and even more than 80% by mass, of the fluorine-based polymer, or 100% of the fluorine-based polymer may be polyvinylidene fluoride polymers.

[0039] Polyvinylidene fluoride polymers are fluorinated polymers containing vinylidene fluoride units. Polyvinylidene fluoride polymers can exhibit high binding strength. Because polyvinylidene fluoride polymers maintain polarity at the vinylidene fluoride units, they have a higher affinity for polar solvents such as N-methyl-2-pyrrolidone (NMP) compared to other fluorinated polymers.

[0040] The polyvinylidene fluoride polymer may be polyvinylidene fluoride itself, or a copolymer of vinylidene fluoride and other monomers. Examples of other monomers include ethylene, propylene, tetrafluoroethylene (TFE), and hexafluoropropylene (HFP). The proportion of vinylidene fluoride units to the total monomer units is preferably in the range of 50 to 100 mol%, and more preferably in the range of 75 to 100 mol%.

[0041] The polyvinylidene fluoride polymer may be polyvinylidene fluoride (PVDF) and its modified forms, polyvinylidene fluoride-hexafluoropropylene copolymer, polyvinylidene fluoride-tetrafluoroethylene copolymer, polyvinylidene fluoride-chlorotrifluoroethylene copolymer, polyvinylidene fluoride-pentafluoropropylene copolymer, etc. The polyvinylidene fluoride polymer may be used alone or in combination of two or more types.

[0042] Polytetrafluoroethylene polymers are fluorine-based polymers containing tetrafluoroethylene units. Polytetrafluoroethylene polymers may be polytetrafluoroethylene (PTFE) or copolymers of tetrafluoroethylene and other monomers. Examples of other monomers include ethylene, propylene, and hexafluoropropylene (HFP). The proportion of tetrafluoroethylene units to total monomer units is preferably in the range of 50 to 100 mol%, and more preferably in the range of 75 to 100 mol%.

[0043] The weight-average molecular weight Mw of the polyvinylidene fluoride polymer and the polytetrafluoroethylene polymer is, for example, 300,000 to 2,000,000, but may also be 500,000 to 1,500,000, or 1,000,000 to 1,500,000. This increases the strength of the positive electrode mixture layer (especially the second positive electrode mixture portion) and makes it less likely for the positive electrode mixture layer to peel off from the positive electrode current collector.

[0044] In this specification, the weight-average molecular weight can be measured using an instrument such as gel permeation chromatography (GPC) for each resin.

[0045] The first positive electrode mixture in the first region and the second positive electrode mixture in the second region may each contain, for example, 0.1 to 10 parts by mass, preferably 0.5 to 2 parts by mass, of binder per 100 parts by mass of positive electrode active material. This provides a sufficient effect to suppress the curvature difference of the first positive electrode mixture and increases the content of positive electrode active material in the positive electrode mixture layer, making it possible to obtain a higher-capacity positive electrode.

[0046] The positive electrode mixture may contain binders other than fluorine-based polymers, but it is preferable that the total amount of fluorine-based polymers accounts for 80% by mass or more, and more preferably 90% by mass or more, of the binders.

[0047] The binder can be separated, for example, by separating the positive electrode mixture layer from the positive electrode and reacting it with an acidic aqueous solution. The acid dissolves components such as the positive electrode active material in the acidic aqueous solution. Subsequently, the acidic aqueous solution is filtered to obtain a residue containing the binder. The binder contained in such a residue can be quantitatively separated using separation devices such as centrifugation, solvent extraction, or column chromatography. The separated components can be analyzed qualitatively and quantitatively using various analytical instruments.

[0048] The positive electrode active material may be, for example, a material that reversibly intercepts and releases lithium ions. The positive electrode active material may also be, for example, a lithium-containing transition metal oxide. Typical examples of lithium-containing transition metal oxides include lithium cobaltate and lithium nickelate, which have a layered, rock-salt crystal structure.

[0049] As the positive electrode active material, for example, a composite oxide containing lithium and a transition metal such as Ni, Co, or Mn can be used. For example, Li ａ CoO ２ Li ａ NiO ２ Li ａ MnO ２ Li ａ Co ｂ Ni １－ｂ O ２ Li ａ Co ｂ M １－ｂO ｃ Li ａ Ni １－ｂ M ｂ O ｃ Li ａ Mn ２ O ４ Li ａ Mn ２－ｂ M ｂ O ４、 LiMPO ４、 Li ２ MPO ４ F (where M is at least one selected from the group consisting of Na, Mg, K, Ca, Rb, Sr, Sc, Y, Ti, Zr, V, Nb, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B) is an example. Here, 0 < a ≤ 1.2, 0 < b ≤ 0.9, and 2.0 ≤ c ≤ 2.3. Note that the value of a, which indicates the molar ratio of lithium, increases or decreases with charging and discharging. In order to satisfy the above conditions, at least one of the first positive electrode mixture and the second positive electrode mixture may be appropriately selected and mixed to use two or more positive electrode active materials.

[0050] Among them, Li ａ Ni １－ｂ M ｂ O ２ Lithium nickel composite oxides represented by (M being at least one selected from the group consisting of Mn, Co, and Al, with 0 < a ≤ 1.2 and 0 < b < 0.7) are preferred. From the viewpoint of increasing capacity, it is more preferable that 0 < b < 0.2 is satisfied. From the viewpoint of crystal structure stability, Li containing Co and Al as M is preferred. ａ Ni １－ｂ Co ｄ Al ｅ O ｃ Or, Li including Co and Mn as M ａ Ni １－ｂ Co ｄ Mn ｅ O ｃ (0 < a ≤ 1.2, 0 < b < 0.2, 0 < d < 0.15, 0 < e ≤ 0.1, b = d + e) ​​is even more preferable.

[0051] The volume-based particle size distribution of the positive electrode active material in the first region may have at least two peaks. For example, the positive electrode active material may be a mixture of a first metal composite oxide having a first particle size distribution with a volume-based median diameter (D50) d1 (hereinafter also referred to as the "first particle group") and a second composite metal oxide having a second particle size distribution with a volume-based median diameter (D50) d2 (hereinafter also referred to as the "second particle group"). The median diameters d1 and d2 may satisfy d1 > d2. The d1 / d2 ratio may be, for example, 2 to 6 or 3 to 5.

[0052] d1 is, for example, 8 μm or more, but may also be 10 μm or more, 11 μm or more, 12 μm or more, or 15 μm or more. Alternatively, d1 may be 30 μm or less, 25 μm or less, or 20 μm or less. As described above, it is preferable that d1 is 8 ≤ d1 (μm) ≤ 30.

[0053] d2 may be 10 μm or less, 8 μm or less, 6 μm or less, or 5 μm or less. From the viewpoint of further improving the charge-discharge cycle characteristics, d2 may be 1 μm or more, or 3 μm or more. As described above, it is preferable that d2 be 1 ≤ d2 (μm) ≤ 10.

[0054] The volume-based median diameter (D50) may be determined by separating the positive electrode active material from the positive electrode mixture layer and measuring it, or by image analysis of a cross-sectional SEM image of the positive electrode mixture layer. Both methods yield approximately the same (without significant difference) median diameter (D50).

[0055] When separating the positive electrode active material from the positive electrode mixture layer, the positive electrode mixture layer may be peeled off the positive electrode (P), immersed in a suitable solvent to dissolve or swell components other than the active material particles, such as dispersants, and separated by centrifugation, one or more times. When the separated sample of positive electrode active material is analyzed with a laser diffraction scattering particle size distribution analyzer, a volume-based particle size distribution can be obtained. The particle diameter of the peak with the largest area in the obtained particle size distribution is the median diameter of either the first particle group or the second particle group, and the particle diameter of the peak with the second largest area is the median diameter of the other. The larger median diameter is d1, and the smaller one is d2. If the peaks overlap, peak separation can be performed by image analysis.

[0056] When performing image analysis of a cross-sectional SEM image of a positive electrode mixture layer, first, the positive electrode mixture layer and the positive electrode current collector are simultaneously cut along the width direction of the positive electrode to obtain a cross-sectional sample of the positive electrode in the thickness direction. At this time, the cross section may be processed with a cross-section polisher (CP) to obtain the cross-sectional sample. Next, the cross section of the positive electrode mixture layer in the cross-sectional sample is observed using a scanning electron microscope (SEM).

[0057] From the contour image of the active material particles in the SEM image, the area enclosed by the contour is determined. The diameter of a circle (equivalent circle) having the same area as the area enclosed by the contour of the active material particles is determined and is taken as the particle size of each particle i. Then, the volume of a sphere having the same diameter as the equivalent circle is considered as the volume Vi of each particle i. By determining the diameter and volume of the equivalent circles for any 100 or more (preferably 1000 or more) particles, a volume-based particle size distribution can be obtained. From the obtained particle size distribution, the median diameters d1 and d2 can be calculated in the same way as when separating the positive electrode active material from the positive electrode mixture layer.

[0058] Examples of conductive additives include carbon materials such as graphite, carbon black such as furnace black and acetylene black, carbon fibers (carbon nanotubes (CNTs), carbon fibers other than CNTs), and graphene. Conductive additives may be used individually or in combination of two or more types.

[0059] As the positive electrode current collector, a non-porous conductive substrate (such as metal foil) or a porous conductive substrate (such as mesh, net, or perforated sheet) can be used. Examples of materials for the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium. The thickness of the positive electrode current collector is not particularly limited, but is preferably 1 to 50 μm, and more preferably 5 to 20 μm.

[0060] [Negative Electrode] The negative electrode comprises a strip-shaped negative electrode current collector. The negative electrode may have a negative electrode current collector and a negative electrode mixture layer or negative electrode active material layer formed (or supported) on all or part of the surface of the negative electrode current collector. The negative electrode mixture layer or negative electrode active material layer may be in the form of a film. The negative electrode mixture layer or negative electrode active material layer is supported on one or both surfaces of the negative electrode current collector.

[0061] The negative electrode mixture layer is composed of the negative electrode mixture. The negative electrode active material layer is composed of the negative electrode mixture or the negative electrode active material. Since the negative electrode mixture contains the negative electrode active material as an essential component, the negative electrode mixture layer may also be called the negative electrode active material layer. The negative electrode active material may be a material that reversibly intercepts and releases lithium ions, a lithium metal, or a lithium alloy. The negative electrode active material layer, which is composed of materials other than the negative electrode mixture, is composed of at least one selected from the group consisting of lithium metal and lithium alloy. The negative electrode mixture layer or the negative electrode active material layer is supported on one or both surfaces of the negative electrode current collector.

[0062] The negative electrode mixture contains a negative electrode active material as an essential component and may contain binders, conductive additives, thickeners, etc., as optional components. Such a negative electrode mixture layer can be formed, for example, by dispersing a negative electrode slurry containing particles of the essential negative electrode active material and optional components in a dispersion medium, applying it to the surface of a negative electrode current collector, and drying it. The dried coating may be rolled if necessary.

[0063] If the negative electrode comprises a negative electrode mixture layer, the negative electrode mixture layer may contain an alloying material. The alloying material contains a phase that reversibly forms an alloy with lithium. The phase that reversibly forms an alloy with lithium may be, for example, silicon (silicon phase). Such a phase exhibits very large expansion and contraction due to charging and discharging. The content of the alloying material in the negative electrode mixture layer may be higher in the negative electrode edge than in the negative electrode core. This makes it easy to increase the expansion rate of the negative electrode at the negative electrode edge compared to the negative electrode core.

[0064] The category of alloying materials includes Si-containing materials, Sn-containing materials, Si-Sn-Si alloys, and Sn alloys. Among these, Si-containing materials are suitable as anode active materials due to their high capacity. Si-containing materials contain a silicon phase. Silicon can reversibly form alloys with lithium. Si-containing materials are materials that can reversibly intercept and release lithium ions.

[0065] The silicon-containing material may be a composite particle comprising a silicon phase and a matrix phase in which the silicon phase is dispersed. The matrix phase may be composed of a material having lithium-ion conductivity. For example, the matrix phase may include at least one selected from the group consisting of a silicon oxide phase and a carbon phase.

[0066] The silicon oxide phase contains Si and O, and may also contain a third element other than Si and O. ２ It may be composed of [a certain material], or it may be composed of lithium silicate, or it may be composed of both of these.

[0067] The silicon-containing composite particles (composite particles including a silicon phase and a matrix phase in which the silicon phase is dispersed) may be in any of the following forms, for example: (a) to (c).

[0068] (a) A silicon phase and silicon dioxide (SiO₂) in which the silicon phase is dispersed. ２ A first composite particle containing the ) phase.

[0069] (b) A second composite particle comprising a silicon phase and a lithium silicate phase in which the silicon phase is dispersed.

[0070] (c) A third composite particle comprising a silicon phase and a carbon phase in which the silicon phase is dispersed.

[0071] Materials other than Si-containing materials include carbon materials, spinel-type lithium titanium oxide, and spinel-type lithium manganese oxide. Among these, carbon materials are preferred. Carbon materials may include graphite, easily graphitizable carbon (soft carbon), and poorly graphitizable carbon (hard carbon). Among these, graphite is preferred because it has excellent charge / discharge stability and low irreversible capacity.

[0072] Graphite refers to a carbon material in which the interplanar spacing d002 of (002) planes, as measured by X-ray diffraction, is, for example, 0.340 nm or less. The crystallite size Lc(002) of graphite, as measured by X-ray diffraction, may be, for example, 5 nm or more, 5 nm or more and 300 nm or less, or 10 nm or more and 200 nm or less. The average grain size of graphite is, for example, 1 μm or more and 30 μm or less.

[0073] When graphite and silicon-containing materials are used in combination, the proportion of silicon-containing material in the negative electrode active material (total of graphite and silicon-containing material) is, for example, 1% to 20% by mass, but may also be 3% to 15% by mass, or 3% to 10% by mass. In this case, a good balance between improved cycle characteristics and increased capacity can be easily obtained.

[0074] Examples of binders include resin materials such as fluororesins like polytetrafluoroethylene and polyvinylidene fluoride (PVDF); polyolefin resins like polyethylene and polypropylene; polyamide resins like aramid resin; polyimide resins like polyimide and polyamideimide; acrylic resins like polyacrylic acid, methyl polyacrylate, and ethylene-acrylic acid copolymer; vinyl resins like polyacrylonitrile and polyvinyl acetate; polyvinylpyrrolidone; polyethersulfone; and rubber-like materials like styrene-butadiene copolymer rubber (SBR). A single binder may be used alone, or two or more may be used in combination.

[0075] Examples of conductive additives include carbon compounds such as acetylene black, carbon fibers (carbon nanotubes (CNTs), carbon fibers other than CNTs), graphene, metal fibers, and metal powders such as aluminum. Conductive additives may be used individually or in combination of two or more types.

[0076] Examples of thickening agents include carboxymethylcellulose (CMC) and its modified forms (including salts such as Na salts), cellulose derivatives such as methylcellulose (cellulose ethers, etc.), and saponified polymers having vinyl acetate units such as polyvinyl alcohol. A single thickening agent may be used alone, or two or more may be used in combination.

[0077] As the negative electrode current collector, a non-porous conductive substrate (such as metal foil) or a porous conductive substrate (such as mesh, net, or perforated sheet) can be used. Examples of materials for the negative electrode current collector include stainless steel, nickel, nickel alloy, copper, and copper alloy. The thickness of the negative electrode current collector is not particularly limited, but is preferably 1 to 50 μm, and more preferably 5 to 20 μm.

[0078] Hereinafter, an example of a secondary battery according to the embodiment of this disclosure will be specifically described with reference to the drawings. The components of the secondary battery example described below can be the components described above. The components of the secondary battery example described below can be modified based on the above description. Furthermore, the matters described below may be applied to the above embodiment. Among the components of the secondary battery example described below, components that are not essential to the secondary battery according to this disclosure may be omitted. Note that the figures shown below are schematic and do not accurately reflect the actual shape and number of components.

[0079] Figure 1 is a schematic cross-sectional view of a secondary battery 10 according to an example of this embodiment. Figure 2 is a schematic plan view of the positive electrode according to an example of this embodiment. Figure 3 is a schematic plan view of the negative electrode according to an example of this embodiment. The actual lengths of the positive and negative electrodes may differ from those shown in the schematic diagrams, and the number of positive electrode leads may also differ.

[0080] The secondary battery 10 may be, for example, a lithium-ion secondary battery or a lithium secondary battery (lithium metal secondary battery). As shown in Figure 1, the secondary battery 10 comprises a non-polar case 11, a wound electrode group 14, a plurality of positive electrode leads 112 made of conductors, a positive electrode terminal 16 made of conductors, an end face current collector plate 19 made of conductors, a negative electrode current collector plate 22 made of conductors, and a sealing plate 23.

[0081] The case 11 is formed in a bottomed cylindrical shape with an opening at one end (the lower end in Figure 1). The case 11 is made of metal. A through hole 12 is formed in the center of the bottom of the case 11 (the upper end in Figure 1), through which the positive electrode terminal 16 is inserted. The case 11 houses an electrolyte (not shown) together with the electrode group 14. A recess 13 is formed near the opening in the case 11, indented radially inward.

[0082] The electrode group 14 has a positive electrode 110 and a negative electrode 120. The electrode group 14 is a wound-type electrode group formed by winding the positive electrode 110 and the negative electrode 120 with a separator (not shown) in between. The electrode group 14 is generally cylindrical in shape as a whole.

[0083] Each of the multiple positive electrode leads 112 has one end connected to the exposed portion 113b of the positive electrode current collector in the first region (positive electrode edge) 113 of the positive electrode 110. The other ends of the multiple positive electrode leads 112 are provided so as to be planted from one end face of the electrode group 14.

[0084] Multiple positive leads 112 are stacked on top of each other and connected to the positive terminal 16 by welding. In this embodiment, there are eight positive leads 112, but the number is not limited to this. Also, in Figure 1, only four of the eight positive leads 112 are shown.

[0085] The material of each positive electrode lead 112 is, for example, stainless steel, aluminum, aluminum alloy, nickel, nickel alloy, etc.

[0086] An insulating member 24 is placed between the electrode group 14 and the bottom of the case 11 to electrically insulate them. The insulating member 24 is made of, for example, an insulating resin. The insulating member 24 may be attached to the bottom of the case 11.

[0087] The positive electrode terminal 16 is located on the opposite side of the electrode group 14, sandwiching the multiple positive electrode leads 112. The positive electrode terminal 16 is inserted through a through hole 12 at the bottom of the case 11 and penetrates the bottom of the case 11. The positive electrode terminal 16 is made of metal, and rivets or the like are used. The positive electrode terminal 16 is insulated from the case 11 by a positive electrode gasket 26 made of insulating material. An insulating plate 25 is placed between the positive electrode terminal 16 and the electrode group 14 to electrically insulate them from each other.

[0088] The positive electrode terminal 16 has a first terminal member 17 that extends both inside and outside the case 11, and a disc-shaped second terminal member 18 that is joined to the first terminal member 17 and exposed to the outside of the case 11. The first terminal member 17 comprises a disc-shaped first portion 17a, a hollow cylindrical second portion 17b that is continuously formed with the first portion 17a and inserted through the through hole 12, and a third portion 17c that extends radially outward from the end of the second portion 17b and to which the second terminal member 18 is joined. The first terminal member 17 is welded to a plurality of positive electrode leads 112 at the first portion 17a by a laser irradiated in the direction from the first terminal member 17 toward the electrode group 14. Thus, the positive electrode terminal 16 is electrically connected to the positive electrode 110 via the plurality of positive electrode leads 112 and functions as an external positive electrode terminal of the secondary battery 10. The first terminal member 17 is an example of a terminal member.

[0089] Of the multiple positive electrode leads 112, at least the positive electrode lead 112 closest to the electrode group 14 (the lowermost positive electrode lead 112 in Figure 1) has a folded portion 112a formed by folding a part of the positive electrode lead 112 (specifically, a part of the tip side), and a part of the laser mark LM formed by the laser is formed thereon. The folded portion 112a is positioned on the opposite side of the electrode group 14, with the insulating plate 25 in between.

[0090] The end face current collector plate 19 is made of metal. The shape of the end face current collector plate 19 is not particularly limited; for example, it may be roughly cross-shaped overall. The end face current collector plate 19 is electrically connected to the negative electrode 120 of the electrode group 14.

[0091] The negative electrode current collector plate 22 is electrically connected to the end face current collector plate 19 via a metal connecting plate 21 (which may be formed in the shape of a ring, for example). Thus, the negative electrode current collector plate 22 is electrically connected to the negative electrode 120. The negative electrode current collector plate 22 and the connecting plate 21 may be welded to each other (for example, by laser welding). The connecting plate 21 and the end face current collector plate 19 may also be welded to each other (for example, by laser welding). The negative electrode current collector plate 22 may be directly connected to the end face current collector plate 19. In this case, the connecting plate 21 is not necessary. The negative electrode current collector plate 22 has one or more injection holes 22a for injecting electrolyte into the case 11. The negative electrode current collector plate 22 is welded (for example, by laser welding) to the recess 13 of the case 11 at its outer edge. Thus, the case 11 is electrically connected to the negative electrode 120 via the negative electrode current collector plate 22, etc.

[0092] The sealing plate 23 seals the opening of the case 11. The sealing plate 23 is made of metal and is generally disc-shaped. The sealing plate 23 is insulated from the case 11 by the negative electrode gasket 27. In this embodiment, the sealing plate 23 is not electrically connected to either the positive electrode 110 or the negative electrode 120 of the electrode group 14, but is not limited to this. The sealing plate 23 has an explosion-proof mechanism (not shown) that activates when the internal pressure of the case 11 exceeds a predetermined value.

[0093] The positive electrode 110 shown in Figure 2 is in its state before being wound as part of the electrode group 14. In Figure 2, arrow Y1 is the winding direction of the positive electrode 110 when manufacturing the electrode group 14, and is the longitudinal direction of the positive electrode 110. Also in Figure 2, arrow Y2, perpendicular to arrow Y1, is the winding axis direction of the positive electrode 110 (i.e., the winding axis direction of the electrode group 14), and is the short-axis direction of the positive electrode 110.

[0094] As shown in Figure 2, the positive electrode 110 has a first region (positive electrode edge) 113 that includes one end 110a in the short direction of the positive electrode 110, and a second region (positive electrode main part) 114 other than the first region 113. The second region 114 is the region from the positive electrode central end 113a of the first region 113 to the other end 110b in the short direction of the positive electrode 110. The ratio of the width (length in the short direction) of the first region 113 to the width (length in the short direction) of the second region 114 is, for example, in the range of 1:15 to 3:4 or 1:12 to 1:6.

[0095] The first region 113 of the positive electrode 110 has an exposed portion 113b of the positive electrode current collector where the positive electrode mixture layer is not placed on the positive electrode current collector, and a first positive electrode mixture portion 113c where the positive electrode mixture layer is placed on the positive electrode current collector. The second region 114 has a second positive electrode mixture portion 114c where the positive electrode mixture layer is placed on the positive electrode current collector.

[0096] The exposed portion 113b of the positive electrode current collector is provided intermittently at multiple locations (for example, 8 locations) along the longitudinal direction of the positive electrode current collector. The exposed portion 113b does not have a positive electrode mixture layer from one end 110a in the short direction of the positive electrode 110 to the second region 114.

[0097] The length of each exposed portion 113b of the positive electrode current collector in the longitudinal direction may be 1% to 10% of the longitudinal length of the positive electrode current collector, and the sum of the lengths of all exposed portions 113b of the positive electrode current collector in the longitudinal direction may be 1% to 20%, 5% to 20%, or 8% to 20% of the longitudinal length of the positive electrode current collector.

[0098] It is desirable that the spacing between the exposed portions 113b of adjacent positive electrode current collectors be as uniform as possible. For example, if the length of the positive electrode current collector is L100 and the number of exposed portions 113b of the positive electrode current collector is n, the spacing between the exposed portions 113b of adjacent positive electrode current collectors may be between 0.8 × L100 / n and 1.2 × L100 / n.

[0099] Each of the exposed portions 113b of the positive electrode current collector is connected to a tab-shaped positive electrode lead 112. Multiple positive electrode leads 112 are bundled together and connected to the first portion 17a of the first terminal member 17.

[0100] The mass W1 per unit area of ​​the positive electrode mixture layer arranged on the surface of the positive electrode current collector in the first positive electrode mixture section 113c may be the same as the mass W2 per unit area of ​​the positive electrode mixture layer in the second positive electrode mixture section 114c. If W1 and W2 are different, the ratio of the difference between W1 and W2 (ΔW) to W1 may be, for example, 4% or less, 3% or less, 2% or less, or 1% or less.

[0101] As shown in Figure 3, the negative electrode 120 has a negative electrode edge portion 123 that faces at least a portion (preferably 70% or more) of the first region 113, and a negative electrode main portion 124 other than the negative electrode edge portion 123. The negative electrode main portion 124 faces at least a portion (preferably 70% or more) of the second region 114. That is, the negative electrode 120 has a negative electrode edge portion 123 that includes one end 120a in the short direction of the negative electrode 120, and a negative electrode main portion 124 other than the negative electrode edge portion 123. The negative electrode main portion 124 is the region from the negative electrode central end 123a of the negative electrode edge portion 123 to the other end 120b in the short direction of the negative electrode 120. The ratio of the width (length in the short direction) of the negative electrode edge portion 123 to the width (length in the short direction) of the negative electrode main portion 124 is, like the positive electrode 110, in the range of, for example, 1:15 to 3:4 or 1:12 to 1:6.

[0102] The other end 120b of the negative electrode 120 in the short direction has an exposed portion 123b of the negative electrode current collector where the negative electrode mixture layer is not disposed on the negative electrode current collector. The exposed portion 123b of the negative electrode current collector is formed along the longitudinal direction of the negative electrode current collector. Therefore, the exposed portion 123b of the negative electrode current collector is exposed at the other end face of the electrode group 14. The exposed portion 123b of the negative electrode current collector is connected to the end face current collector plate 19, for example, by laser welding.

[0103] [Electrolyte] The electrolyte may be a liquid electrolyte (electrolyte solution), a gel electrolyte, or a solid electrolyte. A liquid electrolyte is, for example, an electrolyte solution containing a non-aqueous solvent and a salt dissolved in the non-aqueous solvent. The concentration of the salt in the electrolyte solution is, for example, 0.5 mol / L or more and 2 mol / L or less. The electrolyte solution may contain known additives.

[0104] The gel-like electrolyte comprises a salt and a matrix polymer, or a salt, a non-aqueous solvent, and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs a non-aqueous solvent and gels is used. Examples of polymer materials include fluororesins, acrylic resins, polyether resins, and polyethylene oxide.

[0105] As the solid electrolyte, for example, materials known for use in all-solid-state lithium-ion secondary batteries (e.g., oxide-based solid electrolytes, sulfide-based solid electrolytes, halide-based solid electrolytes, etc.) are used.

[0106] For example, liquid non-aqueous electrolytes are prepared by dissolving a salt in a non-aqueous solvent. The salt is an electrolyte salt that undergoes ion dissociation in the electrolyte, and may include, for example, lithium salts. Various additives may be included in the electrolyte. Electrolytes are usually used in liquid form, but they may also be in a state where their fluidity is restricted by gelling agents or other means.

[0107] Examples of non-aqueous solvents include cyclic carbonate esters, linear carbonate esters, cyclic carboxylic acid esters, and linear carboxylic acid esters. Examples of cyclic carbonate esters include propylene carbonate (PC) and ethylene carbonate (EC). Cyclic carbonate esters having unsaturated bonds, such as vinylene carbonate (VC), may also be used. Cyclic carbonate esters having fluorine atoms, such as fluoroethylene carbonate (FEC), may also be used. Examples of linear carbonate esters include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of linear carboxylic acid esters include methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. The non-aqueous solvent may be used alone or in combination of two or more types.

[0108] Examples of lithium salts include LiClO ４ LiBF ４ LiPF ６LiAlCl ４ LiSbF ６ , LiSCN, LiCF ３ SO ３ LiCF ３ CO ２ LiAsF ６ LiB １０ Cl １０ Examples include lithium lower aliphatic carboxylates, LiCl, LiBr, LiI, borates, and imide salts. Examples of borates include lithium bis(1,2-benzenediolate(2-)-O,O')borate, lithium bis(2,3-naphthalenedioleate(2-)-O,O')borate, lithium bis(2,2'-biphenyldiolate(2-)-O,O')borate, and lithium bis(5-fluoro-2-oleate-1-benzenesulfonic acid-O,O')borate. Examples of imide salts include lithium bisfluorosulfonylimide (LiN(FSO) ２ ) ２ ), bistrifluoromethanesulfonate lithium (LiN(CF ３ SO ２ ) ２ ), trifluoromethanesulfonic acid nonafluorobutanesulfonic acid imide lithium (LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ )), bispentafluoroethanesulfonate lithium (LiN(C) ２ F ５ SO ２ ) ２ Examples include the following. A single lithium salt may be used alone, or two or more may be used in combination. The concentration of the lithium salt in the non-aqueous electrolyte is, for example, 0.5 mol / L or more and 2 mol / L or less.

[0109] [Separator] It is desirable to interpose a separator between the positive electrode and the negative electrode. The separator should have high ion permeability and appropriate mechanical strength and insulating properties. As the separator, a microporous thin film, woven fabric, nonwoven fabric, etc., can be used. As the material of the separator, polyolefins such as polypropylene and polyethylene are preferred.

[0110] (Note) The following technologies are disclosed by the above description. (Technology 1) A secondary battery comprising a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the positive electrode and the negative electrode are wound around the separator, the positive electrode comprises a strip-shaped positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector, the positive electrode has a first region including one end in the short direction of the positive electrode and a second region other than the first region, the first region has one or more exposed portions of the positive electrode current collector provided partially along the longitudinal direction of the positive electrode current collector, the exposed portions do not have the positive electrode mixture layer from the one end in the short direction to the second region, the positive electrode mixture layer comprises a positive electrode active material and a binder, and the value of the loop stiffness load L1 of the first region is smaller than the value of the loop stiffness load L2 of the second region. (Technology 2) The secondary battery according to Technology 1, wherein the ratio L1 / L2 of the load value L1 of the loop stiffness in the first region to the load value L2 of the loop stiffness in the second region is 0.5 or more and less than 1.0. (Technology 3) The secondary battery according to Technology 1 or 2, wherein the ratio T1 / T2 of the thickness T1 of the positive electrode mixture layer in the first region to the thickness T2 of the positive electrode mixture layer in the second region is 0.97 or more and 1.03 or less. (Technology 4) The secondary battery according to any one of Technology 1 to 3, wherein the electrode plate density D1 in the first region is smaller than the electrode plate density D2 in the second region. (Technology 5) The secondary battery according to any one of Technology 1 to 4, wherein the ratio D1 / D2 of the electrode plate density D1 in the first region to the electrode plate density D2 in the second region is 0.7 or more and less than 1.0. (Technical 6) The secondary battery according to any one of Technical 1 to 5, wherein the mass M1 of the positive electrode mixture layer disposed per unit area of ​​the surface of the positive electrode current collector in the first region is smaller than the mass M2 of the positive electrode mixture layer disposed per unit area of ​​the surface of the positive electrode current collector in the second region. (Technical 7) The secondary battery according to any one of Technical 1 to 6, wherein the ratio M1 / M2 of the mass M1 of the positive electrode mixture layer disposed per unit area of ​​the surface of the positive electrode current collector in the first region to the mass M2 of the positive electrode mixture layer disposed per unit area of ​​the surface of the positive electrode current collector in the second region is 0.7 or more and less than 1.0.(Technical 8) A secondary battery according to any one of Technical 1 to 7, wherein the volume-based particle size distribution of the positive electrode active material in the first region has at least two peaks.

[0111] The present invention will be described in detail below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0112] <Example 1> [Preparation of the positive electrode] <First positive electrode slurry> An appropriate amount of NMP was added to the first positive electrode mixture to obtain the first positive electrode slurry. The first positive electrode mixture used a mixture of lithium-containing composite oxide, which is the positive electrode active material, carbon black, which is a conductive additive, and a binder. In the positive electrode mixture, the mass ratio of lithium-containing composite oxide, carbon black, and binder was 98:1:1.

[0113] Lithium-containing composite oxides include LiNi ０．８ Co ０．１ Mn ０．１ O ２ I used it.

[0114] Polyvinylidene fluoride (weight-average molecular weight Mw = approximately 1,000,000) was used as the binder.

[0115] <Second Positive Electrode Slurry> A second positive electrode slurry was obtained in the same manner as the first positive electrode slurry. Here, the density of the positive electrode mixture is changed by changing the amount of each slurry applied to the positive electrode current collector. For example, by applying each slurry in different amounts while maintaining the same thickness, positive electrode mixtures with different densities can be formed.

[0116] <Formation of the first and second positive electrode mixture sections> The first positive electrode slurry and the second positive electrode slurry were applied to both sides of the aluminum foil, which is the positive electrode current collector, to a predetermined thickness. The coating was dried and rolled to form a positive electrode mixture layer comprising the first and second positive electrode mixture sections, thereby obtaining a positive electrode as shown in Figure 2. Specifically, the first positive electrode slurry was intermittently applied to one end of the aluminum foil in the short direction along the longitudinal direction of the positive electrode current collector to a predetermined thickness, and the second positive electrode slurry was applied to the remaining part of the positive electrode current collector to the same thickness. The coating was dried and rolled to form the first positive electrode mixture section (electrode plate density D1 = 3.36 g / cm³). ３ ) a first region and a second positive electrode mixture portion (electrode plate density D2 = 3.5 g / cm³３ A positive electrode was formed comprising a second region having D1 / D2 = 0.96. The mass M1 of the positive electrode mixture layer disposed per unit area of ​​the surface of the positive electrode current collector in the first and second regions was 240 g / m² per side. ２ And M2 at 250 g / m² per side. ２ (M1 / M2 = 0.96) was assumed. Eight exposed portions of the positive electrode current collector were provided in the first region, and a positive electrode lead was attached to each exposed portion.

[0117] The width of the first region (length in the shorter direction) was set to 12 mm, and the width of the second region (length in the shorter direction) was set to 62 mm (the ratio of the length in the shorter direction of the first region to the length in the shorter direction of the second region is 1:5.2).

[0118] The sum of the lengths of the exposed portions of the eight positive electrode current collectors in the longitudinal direction is 10% of the longitudinal length of the positive electrode current collector.

[0119] If the length of the positive electrode current collector is L100 = 720 mm, and the number of exposed parts of the positive electrode current collector is n = 8, then the distance between adjacent exposed parts is L100 / 8 = 90.

[0120] [Fabrication of the negative electrode] SiO₂, the active material of the negative electrode ｘ (x = 1.0) A suitable amount of water was added to a negative electrode mixture containing graphite as the negative electrode active material, styrene-butadiene copolymer rubber (SBR) as a binder, and carboxymethylcellulose (CMC) as a thickener to obtain a negative electrode slurry. In the negative electrode mixture, SiO ｘ The mass ratio of graphite, SBR, and CMC was set to 5:93:1:1.

[0121] A negative electrode slurry was applied to both sides of a copper foil, which served as the negative electrode current collector, to a predetermined thickness, dried, and rolled to form a negative electrode mixture layer, resulting in the negative electrode shown in Figure 3. Specifically, the negative electrode slurry was uniformly applied to the surface of the copper foil to a predetermined thickness, dried, and rolled to simultaneously form the first negative electrode mixture portion and the second negative electrode mixture portion. However, a portion of one end of the negative electrode current collector was left exposed. The thickness of the negative electrode mixture portion was appropriately varied according to the thickness of the positive electrode mixture layer.

[0122] [Preparation of electrolyte] Vinylene carbonate (VC) was added to a non-aqueous solvent obtained by mixing ethylene carbonate (EC) and dimethyl carbonate (DMC) at a volume ratio of 1:3, and LiPF ６ was dissolved to prepare an electrolyte. The content rate of VC with respect to the whole electrolyte was set to 5% by mass. The concentration of LiPF ６ in the electrolyte was set to 1.5 mol / L.

[0123] [Manufacture of secondary battery] In an inert gas atmosphere, a positive electrode and a negative electrode were wound through a separator (a microporous membrane made of polyethylene) to produce an electrode group. The electrodes were stacked such that the positive electrode edge portion was disposed on one end face side of the electrode group and the exposed portion of the negative electrode current collector was disposed on the other end face side of the electrode group. Using the electrode group and the electrolyte, a cylindrical lithium ion secondary battery (Battery A1) as shown in FIG. 1 was completed.

[0124] A plurality of positive electrode leads were bundled and electrically connected to the first portion of the first terminal member. The exposed portion of the negative electrode current collector was connected to the end face current collecting plate by laser welding, and the end face current collecting plate was electrically connected to the negative electrode current collecting plate through a connection plate.

[0125] <Example 2> The mass M1 of the positive electrode mixture layer disposed per unit area of the surface of the positive electrode current collector in the first region was changed to 220 g / m ２ per side (M1 / M2 = 0.88), and the electrode plate density of the first positive electrode mixture portion was changed to 3.08 g / cm ３ (D1 / D2 = 0.88). Except for this change, a battery A2 was manufactured in the same manner as in Example 1.

[0126] <Example 3> In the preparation of the first positive electrode slurry, as the lithium-containing composite oxide, 70 parts by mass of LiNi with a D50 value of 10 μm ０．８ Co ０．１ Mn ０．１ O ２ and 30 parts by mass of LiNi with a D50 value of 3 μm ０．８ Co ０．１ Mn ０．１ O ２ were mixed and used, and the electrode plate density of the first positive electrode mixture portion was 3.5 g / cm ３Battery A3 was manufactured in the same manner as in Example 1, except that (D1 / D2 = 1.0) was changed and the mass of the positive electrode mixture layer per unit area was made the same as that of the second positive electrode mixture layer. The masses M1 and M2 of the positive electrode mixture layer per unit area on the surface of the positive electrode current collector in the first and second regions were both set to 250 g / m² per side. ２ (M1 / M2 = 1.0)

[0127] <Comparative Example 1> Battery B1 was manufactured in the same manner as in Example 1, except that the mass of the positive electrode mixture layer per unit area in the first positive electrode mixture section was the same as that of the second positive electrode mixture layer (M1 / M2 = 1.0).

[0128] [Evaluation] (Loop Stiffness) Ten pieces each of electrode plate samples (ribbon-shaped, 10 mm wide x 100 mm long) from the first region and the second region were cut from the positive electrodes of the examples and comparative examples. Loop stiffness was measured using the measurement method described above, and the average value of the 10 L1 and L2 values ​​was calculated.

[0129] (Cycle Characteristics) Ten secondary batteries each of the example and comparative example were prepared. They were charged at a constant current of 1C in a temperature environment of 25°C. When the battery voltage reached 4.2V, constant voltage charging was performed until the charging current was 0.02C. After a 10-minute rest, constant current discharge at 1C was performed until the battery voltage reached 2.5V. This charge-discharge cycle was performed 300 times. The ratio of the discharge capacity at the 300th cycle to the initial discharge capacity was calculated as the capacity retention rate, and the average value of the 10 batteries was calculated. Table 1 shows the relative values ​​when the average capacity retention rate of Comparative Example 1 is set to 100. A larger relative value indicates better cycle characteristics.

[0130]

[0131] Table 1 shows that cycle characteristics improve significantly when condition A is met. In particular, high cycle characteristics are obtained when L1 / L2 is between 0.6 and 0.9, and even between 0.6 and 0.8.

[0132] The secondary battery described herein is useful as a primary power source for mobile communication devices, portable electronic devices, electric vehicles, and the like.

[0133] Although the present invention has been described in relation to preferred embodiments at present, such disclosure should not be interpreted restrictively. Various modifications and alterations will undoubtedly become apparent to those skilled in the art in the field to which the invention pertains by reading the above disclosure. Accordingly, the appended claims should be interpreted as encompassing all modifications and alterations without departing from the true spirit and scope of the invention.

[0134] 10: Secondary battery 11: Case 12: Through hole 13: Recess 14: Electrode group 16: Positive terminal 17: First terminal member (terminal member) 17a: First part 17b: Second part 17c: Third part 18: Second terminal member 19: End face current collector plate 21: Connecting plate 22: Negative current collector plate 22a: Injection hole 23: Sealing plate 24: Insulating member 25: Insulating plate 26: Positive gasket 27: Negative gasket

[0135] 110: Positive electrode 110a One end 110b Other end 112 Positive electrode lead 112a: Folded portion 113 First region (positive electrode edge) 113a Central end of positive electrode 113b Exposed portion of positive electrode current collector 113c First positive electrode mixture portion 114 Second region (positive electrode main portion) 114c Second positive electrode mixture portion

[0136] 120: Negative electrode 120a One end 120b Other end 123 Negative electrode edge 123a Negative electrode central end 123b Negative electrode current collector exposed part 123c First negative electrode mixture part 124 Negative electrode main part 124c Second negative electrode mixture part

Claims

1. A secondary battery comprising a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the positive electrode and the negative electrode are wound around the separator, the positive electrode comprises a strip-shaped positive electrode current collector and a positive electrode mixture layer disposed on the positive electrode current collector, the positive electrode has a first region including one end in the short direction of the positive electrode and a second region other than the first region, the first region has one or more exposed portions of the positive electrode current collector provided partially along the longitudinal direction of the positive electrode current collector, the exposed portions do not have the positive electrode mixture layer from the end in the short direction to the second region, the positive electrode mixture layer comprises a positive electrode active material and a binder, and the value of the loop stiffness load L1 of the first region is smaller than the value of the loop stiffness load L2 of the second region.

2. The secondary battery according to claim 1, wherein the ratio L1 / L2 of the value of the loop stiffness load in the first region to the value of the loop stiffness load in the second region is 0.5 or more and less than 1.

0.

3. The secondary battery according to claim 1, wherein the ratio T1 / T2 of the thickness T1 of the positive electrode mixture layer in the first region to the thickness T2 of the positive electrode mixture layer in the second region is 0.97 or more and 1.03 or less.

4. The secondary battery according to claim 1 or 2, wherein the electrode plate density D1 in the first region is smaller than the electrode plate density D2 in the second region.

5. The secondary battery according to claim 1 or 2, wherein the ratio D1 / D2 of the electrode plate density D1 in the first region to the electrode plate density D2 in the second region is 0.7 or more and less than 1.

0.

6. The secondary battery according to claim 1 or 2, wherein the mass M1 of the positive electrode mixture layer disposed per unit area of ​​the surface of the positive electrode current collector in the first region is smaller than the mass M2 of the positive electrode mixture layer disposed per unit area of ​​the surface of the positive electrode current collector in the second region.

7. The secondary battery according to claim 1 or 2, wherein the ratio M1 of the mass of the positive electrode mixture layer disposed per unit area of ​​the surface of the positive electrode current collector in the first region to the mass M2 of the positive electrode mixture layer disposed per unit area of ​​the surface of the positive electrode current collector in the second region, M1 / M2, is 0.7 or more and less than 1.

0.

8. The secondary battery according to claim 1 or 2, wherein the volume-based particle size distribution of the positive electrode active material in the first region has at least two peaks.

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